Many modern digital wireless communications systems make use of a data encoding technique known as direct sequence spread spectrum (DSSS) to spread data that is to be transmitted across a larger than necessary amount of spectral bandwidth to help improve the communication system's performance in terms of interference and noise. One particular type of DSSS communications system makes use of orthogonal codes to make communications channels orthogonal to one another. By making the communications channels orthogonal to each other, the communications channels do not interact and interference is reduced. Communications systems making use of orthogonal codes are commonly referred to as code division multiple access (CDMA) communications systems.
Later day CDMA communications systems (3G (Third Generation) systems, such as CDMA2000 and UMTS) permit the existence of high-speed reverse link channels (channels from a mobile station to a base station, for example). The high-speed reverse link channels permit the creating of high-speed data links between the mobile station and the base station for use in applications such as Internet access, voice, data, and multimedia applications, and so forth.
However, in the 3G CDMA systems, the reverse links are transmitted using Binary Phase Shift Keying (BPSK) over an In-phase (I) and Quadrature-phase (Q) subchannels and if usage of the subchannels are not balanced, an inordinate amount of transmit power may be applied to one or the other of the two subchannels. A problem that may arise out of an imbalanced system is that a peak-to-average ratio (PAR) of the link may become large. In order to support a large PAR, power amplifiers with sufficient dynamic range must be used. Unfortunately, these power amplifiers tend to be more expensive than power amplifiers with lower dynamic range and they tend to consume more power. Therefore, the overall cost and power consumption of the wireless equipment increases. Unbalanced use of one subchannel over another may lead to other resource constraints, such as lack of available Walsh codes.
A commonly used technique to attempt to evenly distribute the use of the I and Q subchannels is to assign (also interchangeable with the term allocate) certain channels to certain subchannels. For example, in CDMA2000, channels R-DCCH (reverse dedicated control channel), R-PICH (reverse pilot channel), and R-SCH(2) (second reverse supplemental channel) are assigned to the I subchannel while R-FCH (reverse fundamental channel) and several other logical channels are assigned to the Q subchannel.
One disadvantage of the prior art is that the fixed assignment of the channels to certain subchannels does not consider operating conditions, i.e., when the channels are in operation that may skew the subchannel balance. Therefore, the fixed assignment may make an unbalanced situation even worse by forcing a channel onto an already heavily used subchannel.
A second disadvantage of the prior art is that that the subchannels are summed and then spread. A large number of logical channels that need to be summed can negatively impact the dynamic range of the subchannels. This can arise from the fact that analog-to-digital converters with a fixed degree of resolution may be used to convert the signal to be transmitted into a digital format prior to transmission. When a large number of logical channels are summed, the result of the summation may be large. Therefore, the summation may need to be scaled prior to the digital conversion (to ensure that the summation fits within the resolution range of the analog-to-digital converter), and it is the scaling that can result in loss of dynamic range.